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Industrial Water Softener Design & Sizing Calculation Guide for WTP

Rifka Aisyah Calculation, Water Treatment Plant

In this post, I want to share with you water softener design and preliminary sizing calculation of water softener.

What is a Water Softener in Water Treatment Plants?

In industrial Water Treatment Plants (WTP), a water softener is a critical ion-exchange unit designed to eliminate ions that form scale, which are primarily Calcium (Ca²⁺) and Magnesium (Mg²⁺). If left untreated, hard water leads to severe scaling in downstream boilers, cooling towers, and Reverse Osmosis (RO) membranes, drastically degrading thermal efficiency and shortening equipment life. Water softeners utilize Strong Acid Cation (SAC) resin beads saturated with Sodium (Na⁺) ions. As raw water passes through the pressurized resin bed, Ca²⁺ and Mg²⁺ ions displace the Na⁺ ions on the resin functional groups, delivering softened effluent to the plant.

Water Softener Process
Water Softener Process

Key Parameters for Softener Design

Proper industrial water softener sizing requires comprehensive raw water analysis and key operational metrics:

  • Feed Flow Rate (Q): Peak and continuous service flow rate in m³/h (or GPM).
  • Total Hardness (TH): Concentration expressed as mg/L (ppm) as CaCO₃ or grains per gallon (1 grain per gallons = 17.1 mg/L).
  • Resin Operating Exchange Capacity: working capacity of SAC resin under operating conditions, typically 1.0 to 1.6 eq/L (21.8 to 34.8 kilograins/ft³).
  • Service Cycle Duration (t): Operating run time between regenerations, typically designed for 12 to 24 hours.
  • Regeneration Salt Dosage: Mass of NaCl regenerant per volume of resin bed, commonly 96–192 kg NaCl/m³ (6–12 lbs NaCl/ft³).

Step-by-Step Softener Sizing Methodology

To calculate and size an industrial water softener systematically, follow this 4-step:

  • Step 1: Calculate Total Daily Hardness Load: Load (g CaCO₃) = Feed Flow (m³/h) × Operating Hours (h) × Hardness (mg/L or equivalent with g/m3).
  • Step 2: Determine Required Resin Volume (Vr): Resin Volume (Liters) = Total Hardness Load (eq) / Resin Capacity (eq/L).
  • Step 3: Size the Softener Vessel: Determine tank cross-sectional area based on recommended linear service velocity (15–30 m3/h/m2). Ensure a minimum active resin bed depth of 800 mm (31.5 in) and 40–50% freeboard for backwash expansion.
  • Step 4: Size the Brine Tank & Salt Requirement: Salt mass per cycle (kg) = Resin Volume (m³) × Salt Dosage (kg/m³).

Practical Softener Sizing Calculation Example

Design Basis:

  • Feed Flow Rate Q = 10 m³/h
  • Total Hardness = 200 mg/L as CaCO₃
  • Service Cycle = 24 hours
  • SAC Resin Capacity = 1.2 eq/L (60 g CaCO₃/L)
  • Salt Dosage = 150 kg NaCl/m³

Calculation:

  • Total Hardness Load = 10 m³/h × 24 h × 200 g/m³ = 48,000 g CaCO₃
  • Resin Volume (V_r) = 48,000 g / 60 g/L resin = 800 Liters
  • Vessel Geometry
    • Using linear service velocity 20 m3/h/m2, area = 10 m3/h / 20 m3/h/m2 = 0.5 m2
    • Vessel diameter = (Area × 4 / pi)5 × 1000 = 798 mm ~ 800 mm
    • Bed depth = 1000 mm (selected) + 50% freeboard = 1500 mm
  • Salt Mass per Cycle = 0.80 m³ resin (800 L resin) × 150 kg/m³ NaCl = 120 kg NaCl

I hope you find this simple post useful!

 

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